In the periodical "Der Nahverkehr" ["Local Traffic"], No. 4, 1986, a soot filter of this generic type is described under the headline "Wickelru filter fur Stadtomnibusse in der Erprobung im Verkehrsbetrieb" ["Wound Soot Filter for City Buses Tested in Traffic"]. The soot filter contains a number of tubes in a housing, which are closed at one, and at the other end are welded in gas-tight fashion into a perforated plate that is inserted in a sealed manner into the housing of the soot filter.
The tubes are perforated at their circumference and serve as supports for ceramic yarn, which is wound onto the tubes in the form of cylindrical cross-wound bobbins. The exhaust gas produced by the Diesel engine must necessarily flow through the coils because of the arrangement of the coils of ceramic fibers, and the soot particles contained in the flow of exhaust gas then stick to the ceramic fibers of the coils.
To prevent the flow of exhaust gas, which exhibits relatively very high pressure peaks, from displacing the windings in the coils, the coils must be wound relatively firmly. However, this reduces the effective surface area of the ceramic yarns used for catching the soot particles, because only those regions of the yarn that come into direct contact with the exhaust gas can participate in the soot filtering. Portions of the yarn surface that are covered by adjacent layers are ineffective.
In the known soot filter, if a break occurs in the fiber in a layer of yarn located near the outer surface, the coil loses its internal coherence and is unraveled by the flow of exhaust gas. The yarn on the coil may possibly be removed from the tube and shifted to other regions of the soot filter, where it reroutes the flow path of the exhaust gas. The regions on intact coils that are covered by the yarn material of the unraveled coil become increasingly useless in exhaust gas cleaning because of the increased flow resistance, so that a very large proportion flows through the tube of the soot filter where the filter material has been lost, because the flow resistance is relatively lowest there. Once this malfunction occurs, the soot filter is virtually ineffective any longer.
In the known arrangement of the filter material, the danger of a yarn breakage, particularly in the outer layers, is not inconsiderable. These layers are in fact set into vibration by the exhaust gas which flows highly discontinuously, and moreover major strains arise inside the coil as a result of the change in temperature in the filter material. Because of the relatively stiff winding necessary to give the coil its internal cohesiveness, changes in length resulting from temperature changes cannot be compensated for; instead, the yarn is increasingly subjected to tension.
A further difficulty in the known soot filter is that the coils have a certain tendency to come loose, because of the constant heating and cooling, which forms conduits through the filter material, through which the exhaust gas can flow without notably coming into contact with filter material.